1. Field of the Invention
The present invention generally relates to an apparatus and method for optical drive searching, and more particularly to an apparatus and method for an optical drive searching sync pattern.
2. Description of the Prior Art
As optical storing media is further introduced, storing a large amount of data is not as difficult. And since storing data within an optical media in a digital form is better than that in traditional magnetic media recorded in analog form, the data signal can keep longer and will not distort over time.
Philips and Sony set up the earliest standard of the optical media in 1980 (RED BOOK). In order to store different data content, a lot of different standards (such as: YELLOW BOOK, ORANGE BOOK . . . etc.) are established. However, the optical media data formats are almost all the same and most of them are extended based on the earliest standard, the RED BOOK.
Taking a compact disc (CD) as an example of: at first, data signal is modulated by non-return to zero, NRZ (also called eight to fourteen modulations, EFM). Then the data is stored into a compact disc (CD). For 24 bytes data signal, the data is in form of 588 channel bits after EFM. Since EFM is based on clock, T, the data can be rewritten in 588T. Every 588T includes:                Sync pattern: 24+3 unit: channel bits        Control byte: 14+3        Data byte:12*(14+3)        Error correction: 4*(14+3)        Data byte: 12*(14+3)        Error correction: 4*(14+3)Every 588T forms a frame. In a compact disc, every sector has 98 frames and every 75 sector forms a second data signal (as shown in FIG. 1A).        
Similarly, for digital versatile disc (DVD), the data signal is modulated by EFM plus. The data signal is in form of 1488T of a frame, and then the data signal is stored into a digital versatile disc (DVD).
After modulated by EFM, data signal forms different pits long from 3T to 11T. And these nine different length pits (known as pit and land) forms a spiral groove. The optical drive determines and reads the data signal according to the reflection of pits of the spiral groove.
According to the previous said, the data format of an optical disc, it is understood that every frame has a group of 24 bits sync pattern for separating every frame. By the sync pattern, the optical drive determines every frame range and finds and reads data the signal quickly and correctly. For the compact disc (CD), the sync pattern is formed in 11T/11T and exists in every 588T data signal (as shown in FIG. 1B). And for the digital versatile disc (DVD), the sync pattern is formed in 14T/4T and exists in every 1488T data signal.
Referring to FIG. 2, the symbol 2A is shown as the frame signals in the optical disc. When an optical drive reads a disc, it produces the sync found 2B. When the sync found 2B and the sync pattern of the frame signal occurs at the same time, the optical drive generates the sync flag 2C for highlighting the position of the sync pattern. However, the rotational speed of the optical drive is different when reading the outer circle and the inner circle of the disc. Therefore, the timing of the actual sync found (such as: 2D) and the timing of the sync pattern occurs at different timing and a jitter is generated, so as the optical drive generating the sync flag for labeling the sync pattern is inaccuracy. (As shown in FIG. 3) In order to solve this problem, setting a W width sync window 3C (about 30T) tolerates the error between the sync found 3B and the sync pattern 3A. So if the optical drive can detect the sync pattern within the sync window, it can generate the sync flag 3D and read the data signal correctly.
As the sync window is fixed, when the optical disc has been scraped, the optical drive cannot detect the sync pattern correctly and it can be a problem for reading the data signal.
For example: the optical drive reads a disc as shown in FIG. 4. When the disc is scraped or stained, it generates an 11T signal before the position of the sync pattern occurs. Therefore, the signal includes the sync pattern as 11T/11T/11T (as shown in 4A_4). If the sync window has a large window range, the optical drive can misjudge the first half 11T/11T of the 11T/11T/11T as the sync pattern. It generates the wrong sync flag 4C. As the result, It cannot read the correct data signal and cannot detect the following sync pattern. Furthermore, if the sync window has a small window range, the frequency of the phase lock loop, PLL will be floating and tolerates the error between the sync pattern and the sync flag will be decreased. As a consequence, it cannot lock the sync pattern correctly.